A great many constructions have been proposed for the stem portion of the femoral component of a hip prosthesis. This component attaches to the proximal end of the femur, replacing the natural bone termination, and generally carries the ball of the prosthetic hip joint, which is either separately attached or integrally formed with the stem. The stem fits into the intramedullary canal of the femur, which is generally prepared by resection of the bone end, and reaming or broaching a bore to remove a portion of the central bone tissue. In addition to a number of specialized proprietary or modular constructions, these stems may have one of several overall architectures. One of these involves a completely solid stem having a shaped shoulder portion which fits very precisely into a corresponding prepared cavity that is first machined in the end of the proximal femur. Such stems are made in a discrete number of sizes, and during surgery special milling or boring tools are used to form precise cutouts in the femoral spout to accommodate the contour of the prosthesis, which is driven in to an exact fit with the bone, providing a fairly rigid and tightly fitting attachment without cement. Another form of stem is intended to be fixed primarily by setting it in bone cement. Stems of this type may have somewhat smaller dimensions and shoulder portion, allowing a space between the bone opening and the prosthesis to be filled with bone cement. Each of these constructions involves a strong metal stem, which takes over a significant share of the load carried by the femur.
It has long been known that the provision of a fairly rigid metal prosthesis can result in loss of original bone. This occurs even in the absence of disease, because the processes of bone growth and bone resorption both occur continuously. Bone growth tends to increase in response to active strain in the bone itself, whereas resorption occurs normally at a moderate level, and may increase in dependence upon a number of metabolic or hormonal factors of the individual. Since femoral stem components are generally designed to assure that during their implanted life breakage does not occur, most constructions are quite rigid, so that they carry much of the load normally borne by an intact femur. This results in stress shielding. That is, some regions of the bone experience less strain, causing certain areas of the femur to experience lesser growth, leading to a net resorption, or loss of bone mass. Since many hip stems are designed to be driven into a prepared bone canal and intimately connect to the surrounding bone, care must be taken that the prosthesis itself not assume too great a proportion of the bearing load. This problem has been addressed by designing hip stems to have a more flexible bending stiffness in their distal end. The reduction in bending stiffness is achieved, for example by employing thinner bodies or by providing slots in the stem. The latter approach also aids in initial fitting of the device.
Another commonly encountered problem is that the distal portion of the prosthesis may bear against the inner surface of the bone and cause pain. This occurs most commonly when the prosthesis fails to anchor completely to surrounding bone at the shoulder area, or when anchoring bone in the shoulder area degenerates or is resorbed, so that some wobble of the stem shaft occurs with respect to the bone. It may also occur when misalignment of the stem within the bone canal results in excessive pressure in a localized region of contact at the stem's distal end.
In addition to the foregoing effects, various individual reactions or bone conditions may result in less than optimal fixation of either type of existing stem, or may result in bone loss or bone pain after the stem has been implanted for some time. In the latter case, the condition may have causes other than the altered apportionment of load bearing between the stem and the natural bone, so that one cannot expect to eliminate such complications simply by altering the shape or stiffness of the stem/comnponent. Nonetheless, the prosthesis size, stiffness and overall shape for achieving basic mechanical properties, appear to lie at the root of several common problems.
Accordingly it would be desirable to provide a stem component of different mechanical characteristics.